1. Field of the Invention
The present invention relates to a voltage calibration circuit and related liquid crystal device and related liquid crystal device cable of actively detecting a coupling voltage.
2. Description of the Prior Art
The advantages of a liquid crystal display (LCD) include lighter weight, less electrical consumption, and less radiation contamination. Thus, the LCD monitors have been widely applied to various portable information products, such as notebooks, PDAs, etc. The LCD monitor alters the alignment of liquid crystal molecules to control the corresponding light transmittance by changing the voltage difference between liquid crystals and provides images and produces images with light provided by the backlight module.
A thin film transistor (TFT) LCD monitor has become the most popular display device, so far. The function and the structure of the display module and the driving chip are all well-developed. Please refer to FIG. 1A, which illustrates a schematic diagram of a prior art TFT LCD monitor 10. The LCD monitor 10 includes a display module 120, a source driver 160, and a gate driver 180.
The display module 120 includes a plurality of parallel data lines D1-Dm, a plurality of parallel gate lines G1-Gn, and a plurality of display units P11-Pmn. The data lines D1-Dm intersect the gate lines G1-Gn, and each of the display units P11-Pmn is disposed at the intersection of a corresponding data line and a corresponding gate line. The source driver 160 and the gate driver 180 generate corresponding gate signals and driving signals, respectively. Each display unit of the display module 120 includes a TFT switch 100 and an equivalent capacitor 140. Each equivalent capacitor has an end coupled to a corresponding data line via a corresponding TFT switch, and another end coupled to a common voltage Vcom (Cs on common). When the TFT switch of a display unit is turned on by a gate signal generated by the gate driver 180, the equivalent capacitor of the display unit is electrically connected to its corresponding data line and can thus receive a driving voltage from the source driver 160. Therefore, the display unit can display images of various gray scales by changing the rotation of liquid crystal molecules based on charges stored in the equivalent capacitor 140.
There exists parasitic capacitance 111 within each display unit. At the moment that the gate lines G1-Gn are turned on or off, a voltage variance has an impact over the display units P11-Pmn. When the gate lines G1-Gn are on, the display units P11-Pmn are charge to the accurate voltages. When the gate lines G1-Gn are off, a negative coupling voltage is generated on the display units P11-Pmn. Since the source driving circuit 160 stops charging, the positive voltage and the negative voltage of the display units P11-Pmn are symmetric to the common voltage Vcom, which is fixed. Therefore, the negative and positive liquid crystal molecules of the display data have the same gray level because they have same rotation volume. However, since the parasitic capacitances are different due to the variations of the LCD panel manufacturing process, this causes the negative coupling voltage on the display units P11-Pmn are no long symmetric to the common voltage Vcom. Further, the inconsistency of the gray levels leads to the flickering.
Please refer to FIG. 1B, which is a waveform of an exemplary display unit in FIG. 1A. In FIG. 1B, when the gate line (e.g. G1) goes from a negative level VGL (e.g. −12V) up to a positive level VGH (e.g. 15V), it represents that the gate line is on, where GND denotes a ground terminal. The source driving circuit 160 charges the equivalent capacitor 140 to a display voltage. When the gate line is off, the gate line drops from the positive level VGH (e.g. 15V) to the negative level VGL (e.g. −12V). At this moment, the equivalent capacitor 140 has a voltage drop (usually around 1V) due to the parasitic capacitance 111. After capacitive coupling, the voltages of the data lines D1-Dm are symmetric to the common voltage Vcom. If the difference of the parasitic capacitance is large for each LCD, the display units P11-Pmn may not be symmetric to the common voltage Vcom after the capacitive coupling.
In order to solve the flickering, a NVM is exploited in the prior art, which adjusts the common voltage Vcom according to the flick. However, an extra writing process has to be added in the manufacturing.